Present rotary internal combustion engines have combustion chambers that are noncircular, an example of which is the Wankel engine that typically uses an ellipsoidal housing to encase its combustion chambers. For such an engine's rotor to form a gas tight combustion chamber with the housing, the tolerances of the rotor and housing, as well as the seals therebetween, must be very small, and thus it is difficult and expensive to machine such engines. Also, these engines do not fully exhaust combustion gases from their combustion chambers, resulting in incomplete burning of fuel during subsequent combustion cycles.
Moreover, such engines have an inherent inefficiency because the force released by burning fuel impinges upon the engine's rotor all along the radius between the engine's crank shaft and the outermost extremity of the rotor. Gas impinging upon the rotor closer to the crank shaft exerts less torque than that distant from the crank shaft, and much of the energy released in combustion cannot contribute to the torque on the crank shaft, wasting considerable energy, and making the engine more difficult to cool because this excess energy must be removed. The extra cooling capacity necessitated by this wasted energy causes a further decrease in energy efficiency because the energy to operate this larger cooling system is expended to carry off heat that has served no useful purpose.